Solid state light sources, such as LEDs, are increasingly popular for replacing incandescent light sources, due in part to their significantly lower energy consumption.
Currently, cost-effective solutions for non-dimmable solid state light sources are widely available; however, the cost of a solid state light source that is compatible with phase-cut dimmers is still significantly higher than an equivalent incandescent lamp. This is particularly true for phase cut dimmable light sources for “high mains voltage” such as 220-240V as used in Europe and Asia: the current drawn by a standard solid state light source, used to replace an incandescent lamp of, for example, 40 W is not enough to ensure that the phase cut dimmer behaves properly; moreover, for forward phase-cut dimmers, the non-resistive input impedance of the converter tends to amplify ringing at dimmer turn-on, resulting in erratic behaviour of the dimmer.
For lower mains voltages, such as the 120V mains applications typical in the US, the impedance level is relatively lower (that is, the current to produce the same power level is relatively higher) and smaller dimmer EMI filter inductances are used (of the order of 100 μH as compared to 1 to 5 mH for 230V mains). It thus is easier to keep the dimmer operating properly with limited hardware expense. Such solutions generally are not universally applicable since they cannot be readily extended to higher mains voltages, and in particular to 220-240V for Europe and Asia.
In order to mitigate the effects of a low input current for 230V mains applications, conventional solid state lighting contains functions that in effect mimic an incandescent load: that is to say, they typically include the following three features, which are illustrated with reference to FIG. 1. FIG. 1 shows the voltage and current waveforms for a forward phase-cut dimmer: the top curve 110 shows the input voltage from a forward phase-cut dimmer; the middle curve 120 shows the input current drawn by a 60 W incandescent light source, and the bottom curve 130 shows the input current drawn by a solid state light source.
Firstly, a resistive damper that damps the ringing immediately following turn-on of a forward phase-cut dimmer, for typically 100 μs, shown at 132 in FIG. 1. The ringing results from the dimmer's EMI filter, consisting of an inductor and a capacitor, and the EMI filter in the solid state light source, consisting of one or more inductors and capacitors. Secondly, an RC latch that, at least until the ringing has damped to an amplitude of a only a few tens of milliamperes (mA), draws additional current, thereby providing a positive offset in the current to prevent the ringing from reversing the input current. Typically, this latching current is required for between 50 μs and 300 μs starting from the dimmer turn-on-moment, that is, across regions 132 and 134 of FIG. 1. This RC latch precludes the dimmer conduction current from being at or around zero for too long—that is, for more than a few tens of μs; were this to occur, the triac which is typically used as the dimmer switching device would stop conducting, causing erroneous behaviour. And thirdly, a bleeder that can draw additional DC-current towards the end of the dimmer conduction phase (136 in FIG. 1) to satisfy the dimmer hold current and keep the input voltage low while the dimmer switch is non-conductive (138 in FIG. 1) but still needs some load. The current to be drawn during the non-conduction time is sometimes loosely called the dimmer reset current.
FIG. 2 shows the voltage and current waveforms for a backward phase-cut dimmer; the top curve 210 shows the input voltage from a backwards phase-cut dimmer, the second curve from the top curve 220 shows the input current drawn by a 60 W incandescent light source, the third 230 and bottom 240 curves show the input current drawn by a two different solid state light sources. It will be appreciated that for a backward phase-cut dimmer, the waveforms will appear mirrored, and the ringing due to the steep dVdt at switch-on of a forward phase-cut dimmer will be absent, relative to a forward phase-cut dimmer.
During the dimmer conduction time 232, the light needs to draw at least some current to track the wave form from the backward phase-cut dimmer, in particular when the phase of the mains signal exceeds 90°. After the dimmer conduction has stopped, shown at 234 and 244, the light needs to draw significant current in order to follow the falling edge of the dimmer signal (the current is required in order to discharge the dimmer EMI filter capacitor that is placed across the dimmer switch). During the dimmer no-conduction time 236 of a backward phase-cut dimmer, the light typically needs to draw some current to charge the dimmer's internal supply.
A simplified schematic of a conventional LED lighting circuit is shown in FIG. 3. The figure shows a lighting circuit 300 for a low impedance lighting application, shown as LEDs 394, supplied from a mains, in this case at 230V, via a dimmer 392. The circuit comprises a series resistor RD at the input to a bridge rectifier BD1. Across the bridge rectifier is a series combination of a latch resistor RL and a capacitor CL. The ringing at turn-on is damped primarily by the series resistor RD at the input and, to a lesser extent, by the latch resistor RL. In order to minimise the losses, the damping resistor is chosen to be low-ohmic, and is typically of the order of 50-500Ω. This is the case wherever in the circuit RD is positioned. The temporary latching current (which is typically of the order of 400 mA) is drawn by the series network of RL and CL; a typical time constant, for which this current is drawn, for 230V systems is of the order of 250 μs. It will be appreciated that for 120V systems, the time constant is much shorter, such as 50 μs.
The lighting circuit include a switched mode converter 315 comprising a switch QSW 310 in series with an inductor L2 320. The switch is controlled by controller 330 and dimmer controller 340, which in some configurations may be part of the switched mode converter 315, although in other configurations it may be considered to be separate as shown. A bleed current is drawn by the power transistor QBLD 350, which is controlled by a bleeder controller 360. Sometimes, in order to distribute the heat dissipation, a bleeder resistor may be used in series with the bleeder switch 360. During dimmer conduction, the bleed current may ramp up to typically 15-50 mA, whereas during dimmer non-conduction, the bleed current is only few mA.
The lighting circuit includes an EMI filter 305, which will be familiar to the skilled person, and comprises an inductor L1 between the output of the bridge rectifier BD, (shown as VRECT) and the switched mode converter input bus rail VBUS. Capacitor C1 and C2 are connected between the ground of the switched mode converter and either end of the inductor respectively.
As is clear from FIG. 3, the circuitry to provide the bleeder, latch and damping functions requires additional components, which may have consequences for any of the cost of, electrical losses in or thermal management of the circuit.